The present invention relates generally to systems for stabilizing alternating current (AC) synchronous generators and more particularly to a scheme for checking the integrity of circuitry employed in the stabilization function.
In large power generator installations, the utilization of fast responding electronic equipment for control of the generator field excitation has lead to undesired stimulation of mechanical and electrical resonances. This problem has been found to be particularly severe when multiple generators are electrically coupled together or when long transmission lines are required to distribute the generator power. The problem is generally overcome by the inclusion of a compensating control element called a power system stabilizer. A Power System Stabilizer (PSS) is an auxiliary control device used with generator excitation systems to increase the damping of the turbine generator to power system electromechanical oscillations. Operation of a PSS is well understood in the industry and the various implementation means have been well documented. One example of such a power system stabilizer is found in U.S. Pat. No. 4,788,653 "Digital Filter For Power System Stabilizer" by B. C. Henderson et al., Nov. 29, 1988, which patent is assigned to the assignee of the present invention and which patent is specifically incorporated hereinto by reference. Examples of systems employing a PSS include U.S. Pat. NO. 4,741,023 "On-Line Test and Diagnostic System for Power System Stabilizer" by R. A. Lawson, issued Apr. 26, 1988, which patent is assigned to the assignee of the present invention and U.S. Pat. No. 4,463,306 "System For Stabilizing Synchronous Machines" by F.P. de Mello et al., issued July 31, 1984, both of which are also incorporated hereinto by reference.
The field winding of the synchronous AC generator is, in the typical case, energized by an exciter which is under the control of an automatic voltage regulator. In many of these systems, the output of the PSS is fed to a summing point within the voltage regulator and serves to modulate the voltage regulator output to control generator oscillations. Some voltage regulators, however, have no such summing point or, if it exists, it may be inaccessible to outside signals. For these types of voltage regulators where the PSS signals cannot be directly applied, it is known that the appropriate signal can be injected by means of a modulator. One example of such a modulator is found in the de Mello et al. patent previously cited. This patent utilizes an autotransformer having taps that are selected by solid state switches. Because of the tap changing nature of the de Mello et al. patent, the result is a stepped change in generator excitation. A more continuous scheme for applying the modulation function may be found in U.S. Pat. No. 4,855,644 "Method and Apparatus For Damping Oscillations Of An AC Generator" by L. J. Lane (issued on Aug. 8, 1989) which is assigned to the assignee in the present invention and which patent is also specifically incorporated hereinto by reference.
The previously cited Lawson patent (4,741,023) includes a diagnostic feature which employs the application of a test signal of known frequency that will not stimulate natural frequencies of the system but will provide a means of determining the gain and phase shift characteristics of the PSS. This diagnostic scheme does not, however, test the integrity of the modulator circuit or any other factors external to the PSS. In view of the fact that this external circuitry, especially the modulator circuit, is very likely to be comprised of analog circuitry, the probability of phase shift and/or gain variations due to factors such as aging is relatively high.
Testing of such a system presents a special problem because of the low frequencies involved; i.e. the frequency shifts of the system in normal operation occur at a rate which is in the range of, approximately, 0.2 to 2.0 Hertz. As such, the proper testing of the circuitry requires considerable periods of times. It is not desirable to remove the stabilization system from the generator since instability could occur and thus an on-line testing method is desirable.